Seismic energy sources can roughly be classified into sources that produce high-energy, low-frequency (say 2 to 30 Hz) acoustic pulses and those that produce low-energy, high-frequency (say 600to 7000 Hz) acoustic pulses. Since the earth attenuates high-frequency signals, only low-frequency sources are used for deep-penetration, low-resolution seismic work. The high-frequency sources are used for shallow-penetration, high-resolution work.
Various low-frequency seismic sources using gas exploders have been proposed in the literature and several such gas exploders have been successfully employed commercially. Perhaps the most widely used gas exploder is of the type described in U.S. Pat. No. 3,235,027 of L. G. Kilmer. This patent and several others of Kilmer describe a DYNOSEIS.RTM. gun.
Kilmer's gas exploder essentially comprises a rigid top positioned over a rigid bottom and a vertical extensible sidewall joins the top and bottom to enclose a chamber therebetween. The chamber accepts a combustible gas mixture which is ignited to produce a combustion in the chamber. The energy from the combustion strikes the bottom and extends the side wall. The bottom moves relative to the top and transmits an acoustic signal into the earth. The gas products of combustion are vented from the combustion chamber by an exhaust valve.
The chief similarities between Kilmer's gas exploder and applicants' detonator are: both include a top and a bottom, both ignite a fuel in a chamber to produce downward movement of the bottom. The chief differences are: applicants use a detonating fuel which has a flame velocity very much greater than the flame velocity of the combustible gas of Kilmer; applicants have no extensible vertical sidewall between the top and bottom, so that their products of detonation can be directly flushed out from the detonation chamber through the cavity which is formed following a detonation, by the volume confined between the disengaged top and bottom plates. Other differences between Kilmer's exploder and applicants' detonator relate to the methods of energization and details of construction which will become apparent from the subsequent description.
For high-resolution profiling used in shallow seismic work, all known commercial acoustic energy sources use transducers for converting electric energy into acoustic energy. Unfortunately, the efficiency of such electrically-operated transducers is very low, on the order of 1% or less. This is so because the primary fuel is typically used to first power a diesel engine that drives an AC alternator whose output is converted into DC energy which is converted by the transducer to acoustic energy. Thus, even a portable electrically-operated transducer requires bulky and heavy auxiliary equipments which make the overall system inefficient and limited to special applications. The main advantage of such electric transducers is that they can be operated at high repetition rates.
Accordingly, it is an important object of the present invention to provide a new and improved high-frequency, high-repetition rate acoustic energy source which is relatively lightweight.
It is another object of this invention to provide such a new and improved high-frequency acoustic source which directly converts energy produced by fuel detonation into acoustic energy.
It is a further object of this invention to provide a new and improved mechanical, high-efficiency, high-frequency, high-repetition rate acoustic source which uses the energy provided by a detonating fuel to mechanically strike a member which transmits an acoustic pulse into the ground.
To obtain a relatively-high repetition rate with a fuel transducer, it is essential that the gas products of detonation be flushed out as fast as possible and this is accomplished in accordance with the invention by providing an interplate cavity of large area which directly communicates with the vent port of the detonation chamber.